The present invention relates to methods and apparatus for inerting the atmosphere adjacent moving product surfaces and more particularly, to methods for curing resin bearing web material passed through a treatment chamber at sppeds of up to 1,000 ft. per minute and greater.
In various industrial applications, it has been found necessary to inert the atmosphere above product surfaces passing through treatment devices. For example, in processes for curing resin materials such as coatings or inks on a moving product surface such as a web material by exposure of the resin materials to ultraviolet radiation, electron beams, or other radiation, it has been found necessary to inert the atmosphere above such web and resin material during curing. The reason for requiring such inerting of such atmosphere when utilizing ultraviolet radiation is that the effectiveness of sensitizers in the resin is substantially reduced in the presence of oxygen. Thus, inerting of an atmosphere above a resin bearing material during curing allows for an efficient cure reaction in that less sensitizer is necessary because less is lost by reaction with atmospheric oxygen and accordingly, extra radiant energy is not required to compensate for otherwise lost sensitizer. In addition to the foregoing, it is known that utilization of an inert atmosphere above a resin bearing material during curing will result in improved surface finishes as the formation of resin-oxide bonds is avoided and better cross linking of polymers will result. Similarly, oxygen also interferes with polymerization of resins during curing with electron beam radiation notwithstanding the absence of sensitizers. Furthermore, inerting of such atmospheres enables a wider selection of prepolymer materials to be utilized.
In a typical curing apparatus, a resin bearing web material is passed through an entrance tunnel into a chamber in which it is exposed to necessary radiation and then emerges through an exit tunnel or the like. The product passed into the tunnel will carry in air simply due to skin or boundary layer drag phenomenon. In the event the product is discrete, such as panels of a product with a finite thickness of 0.5 inch or so, the air dragged into the entrance tunnel is trapped due to the configuration of the leading edge of the panel. With a continuous web which may be coated with a particular material being passed into the entrance tunnel of such a curing apparatus, air is carried into such tunnel by a skin or boundary layer effect only. Therefore, in order to avoid excessive quantities of air from being dragged into a curing or other similar apparatus, the atmosphere of the radiation chamber is inerted and air is commonly swept from the entrance tunnel.
Typically, an inert gas is introduced into tunnels and/or treatment chambers and is caused to flow in a direction countercurrent to the direction of movement of the product surface. Attempts to increase the throughput of such treatment devices by increasing the speed at which the product is passed therethrough have not proven effective because the requisite economy in terms of inert gas can only be attained from reliance upon laminar boundary layer conditions which, in turn, are achieved at low product speeds. It has been found that in prior art inerting processes, high product speeds which do not result in laminar boundary layer conditions have required excessive inert gas consumptions. Accordingly, the efficiency of such prior art inerting systems has been limited and, in practice, the speeds at which many moving products can be treated has been likewise limited by the inability to economically inert the atmosphere thereabove at relatively high product speeds.
Apparatus for curing resin bearing web material in inert atmospheres at limited rates is illustrated in U.S. Pat. Nos. 3,807,052 and 3,936,950. In these systems, an inert gas flow is introduced into the entrance tunnel of a curing apparatus at an angle of at least 45.degree. with respect to the web material and in such a manner as to assure a non-turbulent, non-mixing inert gas flow. The stated effect of this flow is to strip, or physically displace, atmospheric air from the surface of the web material although as indicated in these references, the maximum speed at which such materials can be cured is 1,000 ft. per minute. In general, actual operating speeds are approximately half of such maximum speeds. This technique for inerting atmospheres in curing apparatus requires that the inert gas stream have a velocity countercurrent to web velocity and a magnitude greater than the velocity of the moving web material. This, results in a relatively large quantity of inert gas being swept out the entrance of the curing apparatus and hence compromises the economics of the entire curing operation. U.S. Pat. No. 3,654,459 discloses apparatus for curing resin bearing web material in inert atmosphere wherein a non-turbulent condition is established in the introduced an inert gas. Similar systems are illustrated in U.S. Pat. Nos. 3,676,673 and 3,790,801 and in all of such systems it is noted that the maximum speed at which the web material may be translated and at which resin coatings will be cured is limited to approximately 150-200 ft. per minute. A further curing apparatus wherein resin coatings are cured in inert atmospheres is illustrated in U.S. Pat. No. 2,887,584. However, in none of the foregoing patents is there any structure illustrated or process described which permits curing product surface resin coatings or inks on web materials being translated at speeds of 1000 ft. per minute or greater. Accordingly, the prior art described above exhibits a clear need for apparatus in which moving product surfaces such as resin bearing web materials, can be treated, i.e., cured at speeds of 1,000 ft. per minute and greater in an economical manner.